/*
 * Copyright (C) 2015 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;

import com.google.common.annotations.GwtCompatible;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.util.logging.Level;
import java.util.logging.Logger;

@GwtCompatible(emulated = true)
abstract class InterruptibleTask implements Runnable {
    // These two fields are used to interrupt running tasks. The thread executing the task publishes
    // itself to the 'runner' field and the thread interrupting sets 'doneInterrupting' when it has
    // finished interrupting.
    private volatile Thread runner;
    private volatile boolean doneInterrupting;

    private static final AtomicHelper ATOMIC_HELPER;

    private static final Logger log = Logger.getLogger(InterruptibleTask.class.getName());

    static {
        AtomicHelper helper;
        try {
            helper = new SafeAtomicHelper(newUpdater(InterruptibleTask.class, (Class) Thread.class, "runner"));
        } catch (Throwable reflectionFailure) {
            // Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
            // getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
            // For these users fallback to a suboptimal implementation, based on synchronized. This
            // will
            // be a definite performance hit to those users.
            log.log(Level.SEVERE, "SafeAtomicHelper is broken!", reflectionFailure);
            helper = new SynchronizedAtomicHelper();
        }
        ATOMIC_HELPER = helper;
    }

    @Override
    public final void run() {
        if (!ATOMIC_HELPER.compareAndSetRunner(this, null, Thread.currentThread())) {
            return; // someone else has run or is running.
        }
        try {
            runInterruptibly();
        } finally {
            if (wasInterrupted()) {
                // We were interrupted, it is possible that the interrupted bit hasn't been set yet.
                // Wait
                // for the interrupting thread to set 'doneInterrupting' to true. See
                // interruptTask().
                // We want to wait so that we don't interrupt the _next_ thing run on the thread.
                // Note: We don't reset the interrupted bit, just wait for it to be set.
                // If this is a thread pool thread, the thread pool will reset it for us. Otherwise,
                // the
                // interrupted bit may have been intended for something else, so don't clear it.
                while (!doneInterrupting) {
                    Thread.yield();
                }
            }
        }
    }

    abstract void runInterruptibly();

    abstract boolean wasInterrupted();

    final void interruptTask() {
        // interruptTask is guaranteed to be called at most once, and if runner is non-null when
        // that
        // happens, then it must have been the first thread that entered run(). So there is no risk
        // that
        // we are interrupting the wrong thread.
        Thread currentRunner = runner;
        if (currentRunner != null) {
            currentRunner.interrupt();
        }
        doneInterrupting = true;
    }

    private abstract static class AtomicHelper {
        /**
         * Atomic compare-and-set of the {@link InterruptibleTask#runner} field.
         * 
         * @return true if successful
         */
        abstract boolean compareAndSetRunner(InterruptibleTask task, Thread expect, Thread update);
    }

    private static final class SafeAtomicHelper extends AtomicHelper {
        final AtomicReferenceFieldUpdater<InterruptibleTask, Thread> runnerUpdater;

        SafeAtomicHelper(AtomicReferenceFieldUpdater runnerUpdater) {
            this.runnerUpdater = runnerUpdater;
        }

        @Override
        boolean compareAndSetRunner(InterruptibleTask task, Thread expect, Thread update) {
            return runnerUpdater.compareAndSet(task, expect, update);
        }
    }

    private static final class SynchronizedAtomicHelper extends AtomicHelper {
        @Override
        boolean compareAndSetRunner(InterruptibleTask task, Thread expect, Thread update) {
            synchronized (task) {
                if (task.runner == expect) {
                    task.runner = update;
                }
            }
            return true;
        }
    }
}
